I found myself in need for smaller and specialized floating point types,
where I configure the exact configuration (sign, mantissa, exponent,
bias). See http://en.wikipedia.org/wiki/Half_precision for a half
precision number slated for inclusion in IEEE 754r.
Would it be interesting to add a CustomFloat template to phobos? I'm
thinking along the lines of:
template CustomFloat!(bool sign, uint mantissa,
uint exponent, uint bias)
{
...
}
So half-precision numbers are:
alias CustomFloat!(true, 5, 10, 15) HalfFloat;
There are quite a few details to kink out but this is definitely doable.
Numbers like 24-bit floating point and even 8-bit floating point would
be easy to support too. For now CustomFloat would be intended
exclusively as a compact storage mechanism; only conversion to the
standard floating points would be implemented. Later, maybe we can get
to implement some operations natively at least on machines that support
them in hardware. I wanted to gauge interest in the topic.
Andrei

I found myself in need for smaller and specialized floating point types,
where I configure the exact configuration (sign, mantissa, exponent, bias).
See http://en.wikipedia.org/wiki/Half_precision for a half precision number
slated for inclusion in IEEE 754r.
Would it be interesting to add a CustomFloat template to phobos? I'm
thinking along the lines of:
template CustomFloat!(bool sign, uint mantissa,
uint exponent, uint bias)
{
...
}
So half-precision numbers are:
alias CustomFloat!(true, 5, 10, 15) HalfFloat;
There are quite a few details to kink out but this is definitely doable.
Numbers like 24-bit floating point and even 8-bit floating point would be
easy to support too. For now CustomFloat would be intended exclusively as a
compact storage mechanism; only conversion to the standard floating points
would be implemented. Later, maybe we can get to implement some operations
natively at least on machines that support them in hardware. I wanted to
gauge interest in the topic.

In theory it would be great to have this in Phobos. I say in theory
because I haven't actually been in need of such a thing recently, but
it is definitely handy to have if you're working with high dynamic
range images (like from openEXR), or doing other GPU-related things.
Is that the kind of thing you need them for too? I've heard of using
anything beyond 16-bit and 24-bit floats in the GPU/HDR world, though.
A full generic solution is probably overkill for that.
--bb

I found myself in need for smaller and specialized floating point types,
where I configure the exact configuration (sign, mantissa, exponent, bias).
See http://en.wikipedia.org/wiki/Half_precision for a half precision number
slated for inclusion in IEEE 754r.
Would it be interesting to add a CustomFloat template to phobos? I'm
thinking along the lines of:
template CustomFloat!(bool sign, uint mantissa,
uint exponent, uint bias)
{
...
}
So half-precision numbers are:
alias CustomFloat!(true, 5, 10, 15) HalfFloat;
There are quite a few details to kink out but this is definitely doable.
Numbers like 24-bit floating point and even 8-bit floating point would be
easy to support too. For now CustomFloat would be intended exclusively as a
compact storage mechanism; only conversion to the standard floating points
would be implemented. Later, maybe we can get to implement some operations
natively at least on machines that support them in hardware. I wanted to
gauge interest in the topic.

In theory it would be great to have this in Phobos. I say in theory
because I haven't actually been in need of such a thing recently, but
it is definitely handy to have if you're working with high dynamic
range images (like from openEXR), or doing other GPU-related things.
Is that the kind of thing you need them for too? I've heard of using
anything beyond 16-bit and 24-bit floats in the GPU/HDR world, though.
A full generic solution is probably overkill for that.

Yah, graphics is a big target for such types (and a big pusher for
standardizing them). I need such numbers for different purposes, i.e.
storing large arrays of probability distribution. In that case the range
is [0, 1] and I need to cram those numbers in as little space as
possible. I could definitely use a customized floating point layout.
Note that a fully generic implementation is not overkill; if there are
two layouts to support, you might as well support any.
Andrei

I found myself in need for smaller and specialized floating point types,
where I configure the exact configuration (sign, mantissa, exponent,
bias).
See http://en.wikipedia.org/wiki/Half_precision for a half precision
number
slated for inclusion in IEEE 754r.

754r became official two months ago. It's now IEEE 754-2008, and it
includes 16 bit floats.

Would it be interesting to add a CustomFloat template to phobos? I'm
thinking along the lines of:
template CustomFloat!(bool sign, uint mantissa,
uint exponent, uint bias)
{
...
}
So half-precision numbers are:
alias CustomFloat!(true, 5, 10, 15) HalfFloat;
There are quite a few details to kink out but this is definitely doable.
Numbers like 24-bit floating point and even 8-bit floating point would be
easy to support too. For now CustomFloat would be intended exclusively as
a
compact storage mechanism; only conversion to the standard floating
points
would be implemented. Later, maybe we can get to implement some
operations
natively at least on machines that support them in hardware. I wanted to
gauge interest in the topic.

In theory it would be great to have this in Phobos. I say in theory
because I haven't actually been in need of such a thing recently, but
it is definitely handy to have if you're working with high dynamic
range images (like from openEXR), or doing other GPU-related things.
Is that the kind of thing you need them for too? I've heard of using
anything beyond 16-bit and 24-bit floats in the GPU/HDR world, though.
A full generic solution is probably overkill for that.

standardizing them). I need such numbers for different purposes, i.e.
storing large arrays of probability distribution. In that case the range is
[0, 1] and I need to cram those numbers in as little space as possible. I
could definitely use a customized floating point layout.

Wouldn't it be better to use fixed point for that application? Or are
the numbers distributed uniformly in IEEE space (ie, as many between
0.0025 and 0.005, as between 0.5 and 0.1)?

Note that a fully generic implementation is not overkill; if there are two
layouts to support, you might as well support any.

Yeh, mostly thinking about ASM tweaks to make them fast.
That and handling of denormal numbers. Not sure how regular denormals
are across different precisions of floating point numbers. Been a
while since I looked at how denormals are defined, though. Maybe it's
easy.

Yeah. If it's in the denormal range you can't treat the mantissa and
exponent independently. NaNs are a pain, too.
Actually I think most GPUs don't support denormals, infinity, or NaN.

But writing optimized ASM for math ops is probably not as easy to do
in a generic way. Don's probably going to prove me wrong though. :-)

It's very hairy if you're going to support all the rounding modes.
On processors with SSE2, you can do a load/shift/or sequence to shuffle
the bits of a halffloat[4] into a float[4], so the performance shouldn't
be too terrible for array operations, for example. Need to check for
denormals, though, so it gets ugly.
The amount to shift and the bitmasks should be the only things which
change. Perfect application for mixin asm!
There's no SSE shift instructions, so it would be very slow on earlier
x86 machines.

In theory it would be great to have this in Phobos. I say in theory
because I haven't actually been in need of such a thing recently, but
it is definitely handy to have if you're working with high dynamic
range images (like from openEXR), or doing other GPU-related things.
Is that the kind of thing you need them for too? I've heard of using
anything beyond 16-bit and 24-bit floats in the GPU/HDR world, though.
A full generic solution is probably overkill for that.

standardizing them). I need such numbers for different purposes, i.e.
storing large arrays of probability distribution. In that case the
range is
[0, 1] and I need to cram those numbers in as little space as
possible. I
could definitely use a customized floating point layout.

Wouldn't it be better to use fixed point for that application? Or are
the numbers distributed uniformly in IEEE space (ie, as many between
0.0025 and 0.005, as between 0.5 and 0.1)?

The latter. Relative precision has to be about constant within the whole
range. In fact I could store logprobs in fixed-point format, but that
makes addition slow.
Andrei

Yah, graphics is a big target for such types (and a big pusher for
standardizing them). I need such numbers for different purposes, i.e.
storing large arrays of probability distribution. In that case the range
is [0, 1] and I need to cram those numbers in as little space as
possible. I could definitely use a customized floating point layout.

I'll be damned. That was my exact same use case!
I didn't implement a standard half-precision float, though. Knowing that
I'd never have negative values, my type didn't have a sign bit at all. I
think I had a 11-bit mantissa and a 5-bit exponent... or something like
that.
--benji

I found myself in need for smaller and specialized floating point types,
where I configure the exact configuration (sign, mantissa, exponent,
bias).
See http://en.wikipedia.org/wiki/Half_precision for a half precision
number
slated for inclusion in IEEE 754r.
Would it be interesting to add a CustomFloat template to phobos? I'm
thinking along the lines of:
template CustomFloat!(bool sign, uint mantissa,
uint exponent, uint bias)
{
...
}
So half-precision numbers are:
alias CustomFloat!(true, 5, 10, 15) HalfFloat;
There are quite a few details to kink out but this is definitely doable.
Numbers like 24-bit floating point and even 8-bit floating point would be
easy to support too. For now CustomFloat would be intended exclusively as
a
compact storage mechanism; only conversion to the standard floating
points
would be implemented. Later, maybe we can get to implement some
operations
natively at least on machines that support them in hardware. I wanted to
gauge interest in the topic.

In theory it would be great to have this in Phobos. I say in theory
because I haven't actually been in need of such a thing recently, but
it is definitely handy to have if you're working with high dynamic
range images (like from openEXR), or doing other GPU-related things.
Is that the kind of thing you need them for too? I've heard of using
anything beyond 16-bit and 24-bit floats in the GPU/HDR world, though.
A full generic solution is probably overkill for that.

Yah, graphics is a big target for such types (and a big pusher for
standardizing them). I need such numbers for different purposes, i.e.
storing large arrays of probability distribution. In that case the range is
[0, 1] and I need to cram those numbers in as little space as possible. I
could definitely use a customized floating point layout.
Note that a fully generic implementation is not overkill; if there are two
layouts to support, you might as well support any.

Yeh, mostly thinking about ASM tweaks to make them fast.
That and handling of denormal numbers. Not sure how regular denormals
are across different precisions of floating point numbers. Been a
while since I looked at how denormals are defined, though. Maybe it's
easy.
But writing optimized ASM for math ops is probably not as easy to do
in a generic way. Don's probably going to prove me wrong though. :-)
--bb

I found myself in need for smaller and specialized floating point types,
where I configure the exact configuration (sign, mantissa, exponent,
bias). See http://en.wikipedia.org/wiki/Half_precision for a half
precision number slated for inclusion in IEEE 754r.

I would definitely find that useful. Not so much for runtime calculation
(though I wouldn't object to it) but for serialization.
A few years ago, I implement a half-precision float for Java because I
needed to send a bazillion measurements across a slow connection. The
nature of the data was such that I didn't mind losing precision, as long
as I could send twice as many values.
Having that functionality in D, without having to write it myself, would
be very handy.
--benji